1. The comparison of the structure of two collagen genes suggests that very little or no recombinational events took place in these after they diverged from a common ancestor about 500 x 10 to the 6 years ago. This situation contrasts strongly with the intense recombinational rearrangements which the ancestral gene underwent throughout the period of its assembly. We believe the functions of the collagen proteins were the principal selective force against any recombinational rearrangements of the genes once an optimal size for the polypeptide chain was acquired. 2. Studies on methylation of the chick alpha2(I) collagen gene and its DNase I sensitivity in chromatin from several types of tissues with different levels of collagen synthesis suggest at least three levels of regulation for the expression of this gene. 3. The cloned Alpha2(I) collagen promoter after it has been stably integrated in the genome of mouse cells responds to the same type of down-regulation by oncogenic products as the endogenous Alpha2(I) collagen promoter. 4. Studies with deletions in the LTR promoter of Rous sarcoma virus favor the view that "enhancing sequences" are entry sites for RNA polymerase.